Fiber-reinforced polymer composites are well known and widely used. Polymers of improved strength and increased stiffness can be obtained by the use of an appropriate reinforcing fiber. Probably the most widely used reinforcing fibers are glass, carbon and polyamides such as aramid (or "Kevlar" which is a registered trademark of the E.I. du Pont de Nemours & Co., Wilmington, Del.).
The base polymers used in making reinforced polymer composites such as those described above include a wide range of thermoplastics, such as polystyrene and copolymers thereof, polyamides, polycarbonates, polyetherimide, polyether etherketone (PEEK) and polyesters such as polybutylene terephthalate. These polymers are thermoplastics and are either amorphous or semi-crystalline. They may be called flexible chain polymers, since individual monomer units in the polymer chain are free to rotate with respect to each other so that the polymer chain may assume a random shape. By way of illustration, F. N. Cogswell, Intern Polymer Processing, vol. 1, no. 4, pages 157-165 (1987) discloses carbon fiber reinforced PEEK.
More recently developed are self-reinforced polymer composites comprising long, continuous, predominantly unidirectionally oriented fibers of a melt processable wholly aromatic polyester in a matrix of a thermoplastic flexible chain polymer. Such polymer composites are described in commonly assigned, U.S. Pat. No. 4,728,938 of Avraam Isayev et al., issued Mar. 1, 1988. As described therein, the fibers of the wholly aromatic polyester, which may also be termed a thermotropic liquid crystal polymer (LCP) are long continuous fibers formed in situ by mixing the matrix of base polymer with the wholly aromatic polyester in a suitable mixing and extrusion apparatus, as for example, an extruder-static mixer setup, or a twin screw extruder. Polymer composites specifically disclosed therein are polycarbonate/LCP composites containing from 2 to 20 weight percent of LCP, and polyetherimide/LCP composites containing from 5 to 30 percent by weight of LCP.
U.S. Pat. No. 4,835,047 of Avraam Isayev (the inventor herein) et al describes composites of polyetherimide (PEI) and a wholly aromatic polyester or LCP, in which the LCP content varies from 40 to 95 percent by weight. PEI is an amorphous high performance thermoplastic polymer having a glass transition temperature of 215.degree. C. The LCP is present in fibrous domains in a matrix of PEI. Mechanical properties of these composites in general are superior to those described in U.S. Pat. No. 4,728,938 cited supra. These composites of PEI and an LCP are also described in A. I. Isayev and S. Swaminathan, "Thermoplastic Fiber-Reinforced Composites Based on Liquid Crystalline Polymers," Proceedings of the Third Annual Conference on Advanced Composites, pages 259-267, Sep. 15-17, 1987, Detroit, Mich., published by ASM International.
U.S. Pat. No. 5,006,403 of Avraam Isayev (the inventor herein) describes composites of poly(phenylene oxide) (PPO) and a wholly aromatic polyester or LCP, in which the LCP content carries from about 2.5 to about 90% by weight, based on total polymer weight. The LCP is present in fibrous domains in a matrix of PPO. Mechanical properties of the composites in general are superior to those of the base polymers (PPO), especially at higher LCP loadings (about 50% by weight and higher).